It is important to the understanding of tumor biology to know if tumors and preneoplastic lesions arise from single cells (i.e., from rare events). Convincing evidence from chimeric mice suggests that chemically induced tumors are clonal in origin. X-linked mosaic mice have supported the growth of subcutaneous fibrosarcomas containing both marker phenotypes present in nonneoplastic tissue, suggesting that these tumors may have a multicellular origin, although there are other explanations for these observations. We propose to attempt a resolution of the question: "Are chemically induced tumors and preneoplastic lesions clonal in origin?" in four systems, three of which are new. (1)\We have constructed chimeric rats between congenic strains of rat (PVC-RT1[unreadable]a[unreadable], PVG-RT1[unreadable]b[unreadable], PVG-RT1[unreadable]c[unreadable]), which vary in the expression of class 1 major histocompatibility antigens (RT1A) and whose tissues we can distinguish in frozen section. The livers of these animals have been analyzed using digital quantitation, and a relationship between the number of patches/unit area and the percentage of major haplotype has been described. The thymus shows evidence of stem cell clonal expansion in the cortex and the adrenal cortex appears striped, indicating clonal expansion of primordial cells randomly assorted along the medulla. Liver tumors and preneoplastic lesions induced in such chimeras can be analyzed by serial frozen section when stained with antiRT1 monoclonal antibodies, H+E, or for enzyme alterations. This will allow the origin of cells comprising the lesions to be determined. (2)\The same experiment will be conducted with Mus musculus/Mus caroli chimeras. The origin of the cells comprising these mosaic animals can be determined in tissue sections by in situ hybridization. (3)\Epidermal and subcutaneous tumors will be induced in X-linked mosaic animals (Pgk-1[unreadable]a[unreadable]/Pgk-1[unreadable]b[unreadable]) where the origin of the cells comprising the tumor can be determined electrophoretically. (4)\Epidermal and subcutaneous tumors will be induced in X-linked mosaic mice that have double-variant X-chromosome (Pgk-1[unreadable]a[unreadable] and Hprt[unreadable]a[unreadable]). The origin of the cells can be distinguished electrophoretically. If the tumors and preneoplastic lesions are comprised entirely of cells derived from one or the other of the parental strains that form the mosaic tissue, then one may conclude that they arose from a very small number of cells. Conversely, if the altered areas contain both parental strain cell types, then one may conclude that they arose from a large number of cells. (S)